This invention relates to archery bows and, in particular, to a limb reinforcement for compound archery bows.
A compound bow differs from a long bow in that a block and tackle mechanism is used to bend the bow: pulleys or wheels are attached at the free ends of the limbs to obtain a mechanical advantage in bending the bow. The free end of each limb is forked and a wheel turns within the fork on an axle attached to the ends of the fork. The wheels are eccentrically mounted, enabling one to use a much higher draw weight because the wheels provide a substantial "let off" or reduction in holding force of a drawn bow.
A compound bow is typically made in three pieces: an elongated, rigid handle and a pair of limbs. One end of each limb is attached to respective ends of the handle. A limb is an elongated member having a width greater than its thickness and is typically thinner and wider at the middle than at the ends. Holding a limb in ones hand, it seems unlikely that the limb can flex at the middle as the bow is drawn and even less likely that the ends could be twisted. However, because of the large forces in a compound bow, a limb not only flexes at the middle but bends and twists at the ends as well. The twisting is caused by the torques from the lacing connecting the wheels and limbs.
Compound bows typically have lacing wound from an anchor at one end of a first limb to the wheel at the end of the other limb, to the wheel at the end of the first limb, then back to an anchor at the end of the other limb. The middle span between the wheels is the bowstring for receiving the nock of the arrow. The wheel at the end of each limb has two grooves in its perimeter for receiving the lacing. The grooves in the wheel are spaced along the axle, to which the anchor for the end of the lacing is also attached. Thus, these components are spaced along the axle across the width of the limb.
The substantial forces from the lacing combined with the spacing cause torques on the free ends of the limbs which twist the limbs. Moreover, the torques vary as the bowstring is drawn and released. In general, the tension on the bowstring is greatest when the bow is at rest and least when the bow is fully drawn. The situation is reversed in the rest of the lacing: the tension is least when the bow is at rest and greatest when the bow is drawn. As a result of these changing forces, the ends of the limbs twist one way and then the other each time that the bow is drawn.
The fork at the end of the limb is the weakest part of the limb because material is removed from the central portion of the end to provide clearance for the wheel. Any twist at the end of the limb tends to split the limb at the inner end of the fork. The problem has been recognized in the past and a variety of solutions, described in more detail below in conjunction with the drawings, have been used to reinforce the limb. In general, the proposed solutions reinforced the limb by gluing a plate to the limb at the inner end of the fork. A solution of a different kind was to drill a hole in the limb adjacent the inner end of the fork and to attach a bolt through the hole as a rip stop. While originally thought to be effective, these solutions have proven inadequate as draw weights increase and as archers tighten limb bolts past the maximum draw weight of a bow.
As used herein, "plate" refers to a planar member glued to the outer surface of the fork as a reinforcement and "collar" refers to a reinforcement constructed in accordance with the invention. A plate differs from a collar in that, ignoring thickness, a plate is a two dimensional object while a collar is a three dimensional object. The distinction will be more apparent after considering the detailed description of the invention.
A plate is effective until the twisting of the fork exceeds the peel strength of the adhesive fastening the plate to the limb. Then, although the plate itself did not fail, it is no longer effective since it is not fully attached to the limb. The separation may not be visible until the limb itself cracks.
In view of the foregoing, it is therefore an object of the invention to provide an improved limb reinforcement for compound bows.
Another object of the invention is to provide a limb reinforcement attached to the limb in three dimensions.
A further object of the invention is to provide a three dimensional reinforcement which itself resists twisting.